1. Field of the Invention
A significant percentage of the world's current production of copper, about 15%, derives from a leaching process, by various systems, of minerals with a low copper content, in which the latter is present in an oxydized form insoluble in dilute sulfuric acid. The leaching solutions typically contain from 1 to 5 g/l of copper, in addition to other impurities normally found in these materials, such as iron, arsenic, bismuth, antimony, etc.; the direct electrolysis of these solutions for the purpose of producing pure copper is not practical due to both their dilution and their impurities.
2. Description of the Background
The recovery of copper from these solutions has for a long time occurred by precipitating metallic copper while using scrap iron.
Being less noble than copper, the iron replaces the latter according to the well known reaction: EQU Fe+Cu.sup.2+ .fwdarw.Cu+Fe.sup.2+
This operation, known as cementation, produces a fine copper precipitate requiring a successive refining process by thermal and/or electrochemical means to obtain a copper of pure commercial quality.
Other systems for precipitating copper in the sulfide form have been proposed and applied, based on using hydrogen sulfide or sodium sulfide.
The copper sulfide thus obtained was processed by classic pyrometallurgical systems such as flotation concentration.
Certain ion-exchange systems have been developed in recent years, which utilize specific organic solvents to achieve a selective extraction of copper from dilute aqueous solutions and to recover it in concentrated form from a highly acidic copper sulfate solution.
This solution constitutes the circulating electrolyte of an electrolytic system based on insoluble anodes, which deposits the copper contained in the solution on a cathode composed of a thin copper or stainless steel plate.
The deposited copper fits the purity limits requested by the standards.
The anode of this system is a lead alloy plate which evolves oxygen.
The cell voltage of this system is about 2 V, so that the energy consumption per kg of deposited copper is in the range of 1.9 to 2 kW/h.
This process, commercially known as SX-EW, normally utilizes two product classes as solvents, namely salicylaldoxime and ketoxime, diluted in a petroleum distillate such as kerosene.
This process has gradually replaced the copper precipitation system based on cementation with Fe. Compared to the cementation system, the SX-EW process has the advantage of a lower operating cost (due to the savings on the scrap iron, required in a ratio of 1.5-2 kg per kg of precipitated copper), avoids handling the ferrous sulfate solution resulting from the cementation and produces a finished copper product of the highest commercial quality.
These undoubted advantages are opposed by a higher investment cost, a highly sophisticated system operation and technical problems related to the handling of large volumes of organic substances, which may, if improperly controlled, constitute a potential ecological hazard for the surrounding environment.
In these systems, the ratio of the aqueous phase to the solvent is in fact about 1:1, which means that the production of 50 tons/day of copper requires handling a solvent volume of about 1,000 m.sub.3 /h.
With such volumes involved, the inevitable losses of solvent and diluent containing aromatic compounds, while relatively low compared to the copper produced (about 1 kg of solvent and 10 kg of diluent per ton of copper), release certain substances into the environment which can over the medium term certainly adversely affect the biological processes of the surrounding ecosystem.
The mentioned shortcomings are certainly not the only ones of the SX-EW technology.
The extraction of copper from the sulfate and free sulfuric acid solution occurring in an electrochemical reaction with an insoluble lead anode evolving oxygen presents additional problems from both an ecological and an economical viewpoint.
From an ecological viewpoint the oxygen evolves at the anode in the form of tiny high energy bubbles which break up when they reach the surface of the bath, forming an aerosol foam composed of acidic particles which seriously contaminate the working environment.
Although certain measures have been implemented to attenuate this shortcoming, the problems of acidic mists in the SX-EW electrolysis is ever present, with serious consequences for the health of the employees.
From an economical viewpoint, it must be pointed out that in this kind of technology the cell voltage is high due to the anodic component, and that the energy consumption per unit product is high, i.e. about 2,000 kWh per ton of copper.
Another requirement of the process is the need of maintaining about 100 g/m3 of cobalt as a sulfate in the bath by continuous additions, in order to stabilize the anode surface and prevent particles of the same from being incorporated in the cathode, with the resulting contamination of the product.
Considering the high cost of cobalt, these additions materially affect the cost of production.
In conclusion, the SX-EW process in current use and under development as a system for producing electrolytic copper from oxydized copper minerals, while being considered highly reliable, is not without significant negative aspects.